I was modeling something, and when I tried to undo, it made the "skin", as you could call it disappear, and I can't see what I've already drawn
To view all the undoable steps, go to Edit > Undo History and you can select which step you want to revert to. Blender keeps track of a ton of actions such as selecting / unselecting objects, so that's most likely whats going on.



You can also increase/decrease the number of undo steps that Blender remembers in the User Preferences.